1. Field
This application relates to technology for non-volatile data storage.
2. Description of the Related Art
A variety of materials show reversible resistivity-switching behavior, and as such may be suitable as use for memory elements. One type of material having reversible resistivity-switching behavior is referred to as resistance change memory (ReRAM). Transition metal oxides have been proposed for ReRAM. Upon application of sufficient voltage, current, or other stimulus, the reversible resistivity-switching material switches to a stable low-resistance state, which is sometimes referred to as SETTING the device. This resistivity-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistivity-switching material to a stable high-resistance state, which is sometimes referred to as RESETTING the device. This conversion can be repeated many times. The low resistance state is sometimes referred to as an “on” state. The high resistance state is sometimes referred to as an “off” state. For some switching materials, the initial state is low-resistance rather than high-resistance.
These switching materials are of interest for use in nonvolatile memory arrays. One type of memory array is referred to as a cross-point array, which is a matrix of memory elements typically arranged along x-axes (e.g., word lines) and along y-axes (e.g., bit lines). A digital value may be stored as a memory resistance (high or low). The memory state of a memory cell can be read by supplying appropriate voltages to the bit line and word line connected to the selected memory element. The resistance or memory state can be read as an output voltage of the bit line connected to the selected memory cell. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some switching materials may have more than two stable resistance states.
One theory that is used to explain the switching mechanism is that one or more conductive filaments are formed by the application of a voltage to the memory cell. For example, with a metal oxide switching element, the conductive filaments may comprise one or more chains of oxygen vacancies. The conductive filaments lower the resistance of the memory cell. This initial lowering of the resistance may be referred to as “FORMING.” Application of another voltage may rupture the conductive filaments, thereby increasing the resistance of the memory cell. The rupture of the filaments is sometimes referred to as “RESETTING.” Application of another still another voltage may repair the rupture in the conductive filaments, thereby decreasing the resistance of the memory cell once again. The repair of the rupture of the filaments is sometimes referred to as “SETTING.”
Herein any of the operations of FORMING, RESETTING and SETTING may be considered to be a programming operation. After a programming operation on a group of memory cells, it may be desirable for the group to have a tight resistance distribution. However, some conventional techniques do not achieve a tight resistance distribution. For example, after a programming operation that reduces resistance, some memory cells may have a lower resistance than desired.
Some proposed programming techniques may need a high current level to complete the programming operation. This may require higher voltage and current requirements for the supporting circuitry and increase power consumption.
With some proposed techniques, there may be variations in read current level in a single memory cell from one read to the next. For example, some memory cells may exhibit a 2× variation or larger in read current from one read to the next.